The present invention relates to an exhaust gas purifying apparatus and method for purifying exhaust gas emitted from an internal combustion engine.
In an automotive internal combustion engine or the like, it is required to purify the components of exhaust gas, such as carbon monoxide (CO), nitrogen oxide (NOx), and hydrocarbon (HC) before their emission to the atmosphere.
To meet such a requirement, an exhaust gas purifying apparatus for an internal combustion engine disclosed in Japanese Patent Application Laid-Open No. Hei 5-171929 is known. In this exhaust gas purifying apparatus, there is provided an exhaust system in which an exhaust pipe downstream of a catalyst is branched into a first branch flow path and a second branch flow path and these branch flow paths are again merged into a single flow on the downstream side. Then, an adsorbent for adsorbing the hydrocarbon is provided in the first branch flow path, and an exhaust gas recirculation path is provided downstream of the adsorbent for introducing a part of the exhaust gas to the intake side of the internal combustion engine.
In the exhaust gas purifying apparatus, in the case where the catalyst is kept under non-active conditions as in the case where the internal combustion engine is started in a cold condition, the exhaust gas from the internal combustion engine is caused to flow from the catalyst to the first branch flow path and to a muffler through the adsorbent until the catalyst is activated. Thus, the hydrocarbon (HC) contained in the exhaust gas is adsorbed to the adsorbent.
Thereafter, when the catalyst is heated by the heat of the exhaust gas and the temperature of the catalyst reaches the activation temperature, the exhaust gas purifying apparatus renders the exhaust gas, which has been purified by the catalyst, to flow through both first branch flow path and second branch flow path so that the exhaust gas, which has been introduced in the first branch flow path, may flow through the adsorbent to the exhaust gas recirculation flow path and the exhaust gas, which has been introduced into the second branch flow path, may flow to the muffler.
The exhaust gas introduced into the first branch flow path causes the adsorbent to be heated and removes therefrom the hydrocarbon (HC) adsorbed to the adsorbent. The hydrocarbon (HC) separated from the adsorbent is introduced into the exhaust gas recirculation flow path together with the exhaust gas and recirculated to the intake side of the internal combustion engine.
However, in such an exhaust gas purifying apparatus, there is a fear of problems as follows.
When the recirculation amount of the exhaust gas to the intake side is increased, the combustion temperature of the mixture is lowered so that the combustion rate within the combustion chamber is delayed. As a result, the variation of the combustion period for each cycle is remarkable to cause the increase of the torque variation and to induce an uncompleted combustion or a misfire in the combustion chamber. There is a problem that the fuel consumption rate is degraded and the amount of the discharged unburnt hydrocarbon is increased.
On the other hand, if the recirculation amount of the exhaust gas is decreased, since the amount of the exhaust gas passing through the adsorbent is decreased, it takes a long time to make the temperature of the adsorbent rise. Then, in some cases, it takes a long time to separate the hydrocarbon away from the adsorbent, and the engine is at a standstill before the completion of the separation. In the case where the engine is at a standstill before the completion of the separation, the hydrocarbon is still adhered to the adsorbent, there is a problem that the adsorbing performance is degraded.